In reciprocating piston engines of this kind and especially for separately-lubricated four-stroke engines or the like, it is necessary to ensure for the functionally reliable operation thereof that no impermissible overpressure builds up in the crankcase interior space. It is therefore known to provide the crankcase with a compensating system via which the crankcase overpressure can be reduced, for example, into the atmosphere. An oil mist develops in the crankcase so that corresponding measures must be taken to ensure that the oil does not escape uncontrollably but is held back in the crankcase. For this purpose, complex separating elements are provided which, however, do not always guarantee the desired operation. Furthermore, it has been noted that oil escapes through the pressure compensating system when the crankcase is not in the upright position in the case of portable engines at standstill such as small engines in portable apparatus.